This invention relates generally to atomic absorption spectrophotometers and more particularly to a method and apparatus for determining the zero line in atomic absorption spectrometers of the type which utilize an atomizer and a burner for atomizing the sample.
In an atomic absorption spectrometer, a light source, such as a hollow cathode lamp, is utilized which emits a measuring light beam containing the resonance spectral lines of a desired element. This measuring light beam is passed through a flame burning on a burner and impinges upon a photoelectric detector. The flame is supplied with a fuel gas, e.g. acetylene, and an oxidizing agent, e.g. air or laughing gas. A pneumatic atomizer at the burner is also supplied with oxidizing agent. The atomizer takes in a sample liquid which is entrained by the flow of oxidizing agent and at least partially enters the flame in the form of a fine aerosol. In the flame, the sample liquid is atomized such that the elements contained in the sample form a "cloud of atoms" through which the measuring light beam is passed. The atoms of the looked-for element have a resonance spectrum conforming with the spectral lines of the measuring light beam and absorb the light of the measuring light beam. The attenuation of the measuring light beam is a measure of the amount of the looked-for element in the flame and thus the concentration of the looked-for element in the sample.
The signal obtained at the photoelectric detector of the spectrophotometer must be calibrated or corrected relative to a zero line which is obtained if the sample does not contain the desired element. This zero line can be subjected to drift, for example, due to the fact that the light intensity of the light source changes or the sensitivity of the photoelectric detector changes. It is known from European Patent Publication No. 084,391 to provide a reference path of rays which by-passes the flame so that the radiation by-passing the flame provides a signal at the photoelectric detector which varies only with the drift caused by changes in the light intensity of the light source and by changes in the sensitivity of the detector. This signal can be used to compensate the drift of the signals obtained via the sample path of rays through the flame. During the sample measurement, the radiation continuously passes through the sample path of rays. Between sample measurements, the radiation is passed through the reference path of rays. Movable mirrors are utilized to switch the radiation over from one path of rays to the other upon an external command. The reference measurement is preferably made in the reference path of rays while the flame stabilizes after a change-over from one sample to the other. The measurements in the reference path of rays are stored for use in the drift correction.
This known atomic absorption spectrometer needs complex optical and mechanical means for generating a reference path of rays and for periodic switching between the sample and reference path of rays. Also, the influence of the flame itself upon the zero line is not accounted for. The influence of background absorption is eliminated by an additional measurement using a lamp which emits a continuum, e.g. a deuterium lamp.
It is an object of the present invention to provide a new and improved atomic absorption spectrophotometer device.
Another object of the invention is to provide a new and improved apparatus and method for determining the zero line reference measurement in an atomic absorption spectrophotometer.
A further object of the invention is to provide such an apparatus and method which determines the zero line reference without using a separate reference light path and the precision optical assembly for providing such a reference light path.
A further object of the invention is to provide an apparatus and method for determining the zero line reference which accounts for the effects of the flame upon the zero line measurement.
Another object of the invention is to provide an apparatus and method for determining the zero line reference which is accurate, reliable and economical.
Other objects will be in part obvious and in part pointed out more in detail hereinafter.
Accordingly, the foregoing and related objects and advantages may be obtained in a method for determining the zero line reference is an atomic absorption spectrophotometer of the type having an atomizer and burner for atomizing a sample with a flame receiving a regulated or fixed flow of oxidizing agent comprising the steps of halting the flow of sample to the flame of the burner while maintaining the fixed flow of oxidizing agent to the flame and measuring the light absorption of the flame without sample to define a zero line reference. The flow oxidizing agent to the atomizer is halted to stop the flow of sample to the flame and the supply of oxidizing agent to the burner is increased to compensate for the stoppage of oxidizing agent to the flame via the atomizer. In one embodiment of the invention, the flow of oxidizing agent to the atomizer is selectively by-passed to the flame.
An atomic absorption spectrometer apparatus of the present invention includes a burner for producing a flame for burning a sample for atomic absorption spectrometry. A first regulated supply provides a regulated flow of fuel gas and oxidizing agent to the burner for producing the flame. An atomizer introduces sample to the flame in an aerosol form and is connected to a second regulated supply for providing a regulated flow of oxidizing agent to the atomizer during a sample analysis operation. A flow meter measures the flow of oxidizing agent to the atomizer. A light source generates a measuring light beam and the detector produces a measuring signal based upon the absorption of the measuring light beam in the flame. A valve assembly is connected to the atomizer oxidizing agent supply for selectively halting the flow of oxidizing agent to the atomizer and providing an additional flow of oxidizing agent to the burner during a measurement correction determination operation. A regulator controls the additional flow of oxidizing agent to the burner so that the additional flow is equal to the flow of oxidizing agent to the atomizer during a sample analysis operation. An electronic circuit defines the absorption of the measuring beam during a measurement correction determination operation as the zero line measurement.